For providing sufficient electric power to an electronic apparatus such as a mobile phone or a notebook computer, the electronic apparatus may be electrically connected to a power socket through a transmission cable and a plug or the electronic apparatus may be electrically connected to a computer in order to acquire the electric power. Since the conventional electronic apparatus can only be charged by a wired transmission technology through the transmission cable and the plug, it is inconvenient to use and carry the conventional electronic apparatus, the transmission cable and the plug.
For solving the above drawbacks, a variety of wireless charging devices have been disclosed. When an electronic apparatus is placed on a surface of the wireless charging device, the wireless charging device may transfer electric energy to the electronic apparatus in order to charge the electronic apparatus in a wireless transmission manner. During the process of transferring the electric power from the wireless charging device to the electronic apparatus, the coil assembly of the wireless charging device and the coil assembly of the electronic apparatus should be aligned with each other. Moreover, the distance between the coil assembly of the wireless charging device and the coil assembly of the electronic apparatus has a large influence on the wireless charging performance. That is, if the distance is too long or too short, the wireless charging performance is deteriorated or even the wireless charging operation fails to be normally performed. Moreover, if the wireless charging device is employed in a moving transport tool, a furniture product or any other place prone to vibration, the wireless charging device is easily shifted or damaged. Under this circumstance, the wireless charging performance of the wireless charging device is deteriorated. In other words, it is important to reduce the adverse influence from vibration.
Hereinafter, the structure of a conventional wireless charging device will be illustrated with reference to FIGS. 1 and 2. FIG. 1 is a schematic perspective view illustrating a conventional wireless charging device. FIG. 2 is a schematic perspective view illustrating the inner structure of a conventional wireless charging device.
As shown in FIGS. 1 and 2, the wireless charging device 1 comprises a casing 11, a power circuit 12, a holder 13, four rollers 14, and a first coil assembly 15. The casing 11 comprises an upper cover 11a and a base member 11b. The upper cover 11a comprises a charging platform 11aa. 
The holder 13 is disposed within the base member 11b. The four rollers 14 are located at four corners of the holder 13, respectively. The first coil assembly 15 is disposed on the holder 13.
Hereinafter, the operations of the conventional wireless charging device will be illustrated with reference to FIGS. 1 and 2. When an electronic apparatus with a second coil assembly is placed on the charging platform 11aa, the first coil assembly 15 and the second coil assembly are attracted by and aligned with each other. As the electronic apparatus moves, the first coil assembly 15 is synchronously moved with the second coil assembly through the four rollers 14.
After the wireless charging device 1 is connected with a power source, the power circuit 12 may transfer electric energy from the power source to the first coil assembly 15. When an electric current flows through the first coil assembly 15, a magnetic field is generated. Due to the magnetic field generated by the first coil assembly 15, the second coil assembly generates another electric current to charge the electronic apparatus.
However, in a case that the wireless charging device 1 is operated in an object prone to vibration (e.g. within a vehicle), some drawbacks may occur. For example, since the wireless charging device 1 has no mechanism for maintaining the distance between the first coil assembly 15 and the charging platform 11aa and absorbing the vibration energy from the vehicle body, the first coil assembly 15 may be shaken up and down within the wireless charging device 1 during the vehicle is driven by the user. Under this circumstance, the wireless charging performance is adversely affected. Moreover, if the vibration is too serious, the wireless charging device 1 is possibly damaged.
Therefore, there is a need of providing an improved wireless charging device in order to eliminate the above drawbacks.